In general, for forming a ring pattern on a semiconductor substrate, a dielectric layer, such as boron phosphate silicate glass (BPSG), may be formed on the semiconductor substrate, and then a positive photoresist is formed on the dielectric layer. Next, a photomask shown in FIG. 1 is employed as a mask and then the positive photoresist is exposed and developed to form a patterned photoresist. When the positive photoresist is irradiated by light, the polymer bonding within the positive photoresist will be cut off so that the exposed portion of the photoresist can be removed in a subsequent development step. That is, after the development step, the desired ring pattern 101 is transferred to the positive photoresist. Subsequently, an unprotected portion of the substrate can be etched to a depth during an etching step by using the patterned photoresist as a mask. Finally, the positive photoresist is removed and the desired ring structure is formed.
However, the ring pattern 101 shown in FIG. 1 of the photomask is impacted by the exposure process which causes the inner wall and outer wall of the ring shown in FIG. 1 not to keep a constant distance d. Moreover, in the conventional etching process not only the dielectric layer is etched, but also a portion of the photoresist is etched. Therefore, a thicker photoresist layer is generally required to make sure that an unexpected structure will not result from the etching step due to the completely consumption of the photoresist.
Accordingly, it is desired to provide a method of forming a ring pattern, which overcomes the limitation of the patterning processes and reduces the cost by reducing the thickness of the photoresist.